<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

</body>
<script>
    function UN() {
        // 判断是否存在实例
        if (typeof UN.instance === 'object') {
            return UN.instance;
        }

        // 其它内容
        this.start_time = 0;
        this.bang = 'hello world';

        // 缓存，如果没有这一步，实例不全等
        UN.instance = this;

        // 构造函数隐式返回this
    }
    var u = new UN();
    var u2 = new UN();
    console.log(u === u2);
</script>
</html>